Final answer:
Birdsong varies between species and within a species, and it is used to mark territories and attract mates.
Step-by-step explanation:
The correct statement about birdsong is (D) All of the answers are correct. Birdsongs vary between species and within a species, and they are used for multiple purposes, including marking territories and attracting mates. For example, male birds often sing to establish and defend their territories, while they also use their songs to attract females during the breeding season.